Typically employed aqueous resin emulsions have been prepared by emulsion polymerization of a water sparingly soluble radical-polymerizable monomer, such as styrene, in the presence of an emulsifying agent and a water-soluble radical polymerization initiator. The emulsifying agent used is generally effective for ensuring polymerization stability in emulsion polymerization and for obtaining an aqueous resin emulsion having a small particle size and dispersion stability. The mechanism of the emulsifying agent to increase dispersion stability of the resulting emulsion has not been fully elucidated, but it is thought that an adsorption-desorption equilibrium is established in the emulsion between a part of the emulsifying agent adsorbed onto emulsion particles and the rest of the emulsifying agent present in the emulsion in a free state, and dispersion stability of the emulsion can thus be realized. Therefore, when an aqueous emulsion containing an emulsifying agent is dispersed in a buffer solution or physiological saline, the aforesaid adsorption-desorption equilibrium of the emulsifying agent in the resin emulsion is disturbed, and, as a result, stability of the resin emulsion is reduced, which readily leads to agglomeration or sedimentation of resin particles. In addition, since a polystyrene emulsion comprises hydrophobic resin particles, agglomeration of resin particles readily occurs when it is dispersed in a buffer solution or physiological saline containing an organic solute.
There is known a process of obtaining an aqueous resin emulsion in which a methacrylic ester is emulsion copolymerized with hydrophilic methacrylic acid, 2-hydroxyethyl methacrylate and a polyfunctional monomer for internal cross-linking in the presence of a water-soluble radical polymerization initiator and an emulsifying agent, as disclosed in Polymer, Vol. 19, pp. 867-871 (Aug., 1978). However, the emulsion obtained by this process also contains an emulsifying agent, and, therefore, involves the same problem as mentioned above. Further, the above described emulsion has a tendency that resin particles are readily agglomerated especially upon dispersing in a buffer solution, etc., presumably because the copolymer particles have a hydroxyl group.
For eliminating the above described problem, there has been proposed a process of obtaining an acrylic ester emulsion in the absence of an emulsifying agent as disclosed by Kagaku Gijutsu Kenkyusho Report, Vol. 75, No. 8, p. 341 (1980). However, the resin emulsion obtained by this process is still unsatisfactory in terms of dispersion stability, and, in particular, easily undergoes agglomeration under mechanical shear stresses.
The present invention has been accomplished in order to overcome the above described disadvantages.